1. Field of the Invention
The present invention relates to a heating-type toner-fixing unit in which (a) a pressure roller drives a continuous heat-resistant film by pressing it against a ceramic heater positioned on a heating cylinder and (b) a toner image is fixed on a copying sheet fed between the pressure roller and the heat-resistant film. The present invention particularly relates to a ceramic heater for a toner-fixing unit in which the ceramic heater is positioned on a heating cylinder.
2. Related Background Arts
Toner-fixing units used in fax machines, copiers, printers, and other image-forming machines transfer a toner image formed on a photosensitive drum onto a sheet of paper or another copying sheet before fixing it on the surface of the sheet by concurrent heating and pressing.
These toner-fixing units comprise a heating roller and a plastic pressure roller. One of these units, for example, uses a cylindrical metal roller provided with a halogen lamp or another heat source as the heating roller so that the surface region of the metal roller is heated by the heat source to fix the toner. In addition to this fixing system, another toner-fixing system has been offered and used in recent years which uses a ceramic heater (hereinafter also simply called xe2x80x9ca heaterxe2x80x9d) without using a heating roller. The latter system has been disclosed in published Japanese patent applications Tokukaihei 1-263679, Tokukaihei 2-157878, and Tokukaishou 63-313182, for example. Specifically, these disclosed systems (a) mount a ceramic heater on a plastic support, (b) use a pressure roller to press a heat-resistant film against the ceramic heater so that the heat-resistant film can move at the same speed as the peripheral speed of the pressure roller to feed a sheet of paper, and (c) fix a toner image on the surface of the sheet. The heat-resistant film is composed of a material comprising heat-resistant plastic, metal, or both.
The latter fixing system, using a toner-fixing unit comprising a ceramic heater and a heat-resistant film, has a heater significantly smaller in thermal capacity than that of the former system, using a metal roller. Therefore, the latter system can reduce the power consumption and eliminate the preheating of the heater after the power is supplied, so it is advantageous in having an excellent quick-start property.
FIG. 1 schematically shows a modified latter fixing system in which a continuous heat-resistant film revolves around a heating cylinder. In FIG. 1, a ceramic heater 1 is positioned on a heating cylinder 2. A continuous heat-resistant film 3 revolves on the periphery of the heating cylinder 2. A pressure roller 4 has a rubber layer or another elastic layer formed on its periphery and its rotation revolves the heat-resistant film 3 at the same speed as its peripheral speed. The heating cylinder 2 and the pressure roller 4 are pressed against each other by springs (not shown in FIG. 1) provided at the fixed portions of both of their ends. This pressure deforms the elastic layer on the periphery of the pressure roller 4, forming a nip portion 5 having a width of W. A toner image 6a is formed on a copying sheet 6 such as a sheet of paper. The copying sheet 6 is fed between the heat-resistant film 3 and the pressure roller 4 each rotating in the direction of the arrow. The toner image 6a on the copying sheet 6 is heated and pressed at the nip portion 5 to be fixed as an image 6b. 
The ceramic heater 1 attached to the heating cylinder 2 has a structure shown schematically in FIG. 2, for example. FIG. 2 is a plan view viewed from the downside. In FIG. 2, a ceramic base material 11 supports (a) one or more heating elements 12 provided at the face (the fixing face) where the ceramic heater 1 is positioned opposite to the pressure roller 4 via the heat-resistant film 3 and (b) current-feeding electrodes 13 for supplying electric power to the heating elements 12. The ceramic base material 11 generally has a shape of a thin rectangular flat plate as a whole. Generally, the heating elements 12 are formed at the fixing-face side of the ceramic heater 1, and an overcoat glass layer is formed on the heating elements as a protective layer. The glass layer not only ensures electrical insulation but also protects the heating elements 12 and other members against the sliding contact with the heat-resistant film 3.
Alumina is generally used now as the ceramic base material 11 of the foregoing ceramic heater 1. The ceramic heaters using alumina base materials in this image-fixing system have a fixing rate of 6 to 16 ppm. The unit xe2x80x9cppmxe2x80x9d is the abbreviation of xe2x80x9cpapers per minutexe2x80x9d and signifies the number of sheets of paper in A4 size (210xc3x97297 mm) fed in a minute. The foregoing fixing rate, however, no longer satisfies the market""s requirement. The market now requires to increase the rate to 24 ppm or more.
In a ceramic heater in this image-fixing system, a voltage of 100 or 200 V is applied to the heating elements to produce a Joule heat of hundreds of watts or more. This heat raises the temperature to about 200xc2x0 C. in 2 to 6 seconds. In this rapid heating process, ceramic heaters using alumina base materials pose a problem of fracture caused by heat shock. When the fixing rate is increased, the time for transferring the heat from the heater to a sheet of paper (a copying sheet) is shortened. This requires the increase in the amount of heat to be supplied from the heater to the copying sheet per unit time, because the toner fixing requires a certain amount of heat. As a result, the increase in the fixing rate tends to increase the heat shock applied to the heater, increasing the percentage of heater fracture.
In order to cope with this problem, ceramic heaters using aluminum nitride, which has excellent resistance to heat shock, as the base material have been disclosed in published Japanese patent applications Tokukaihei 9-80940 and Tokukaihei 9-197861. Tokukaihei 9-80940 improves the temperature responsivity of the heater by exploiting the fact that aluminum nitride has a higher thermal conductivity than alumina. Tokukaihei 9-197861 intends to improve the fixing quality, to increase the possibility of high-speed printing, and to reduce the power consumption by utilizing the highly heat-conductive quality of aluminum nitride.
As stated above, the use of aluminum nitride as the base material of the ceramic heater can solve the problem of heater fracture. However, further increase in the fixing rate poses another problem of a reduction in the durability of the heat-resistant film (hereinafter also simply called xe2x80x9cfilmxe2x80x9d) revolving around the heating cylinder provided with the ceramic heater.
As mentioned previously, the conventional ceramic heater has a shape of a rectangular flat plate as a whole. Consequently, the continuous heat-resistant film, which is tubular when viewed from the side, is pressed against the flat fixing face provided on the heating cylinder by the pressure roller and deformed to a flat shape at this position, the nip portion. This deformation accompanied by the high-speed revolution applies a heavy load to the film and may cause the film to fracture at an early stage. In particular, a fixing rate exceeding 24 ppm increases this tendency. This problem has been an obstacle for increasing the fixing rate in a system using a heat-resistant film and a ceramic heater.
Considering the above-described problems, an object of the present invention is to offer an improved ceramic heater for a fixing system using a heat-resistant film and a ceramic heater. The ceramic heater is characterized by the following features:
(a) it reduces the degree of deformation of the heat-resistant film to lighten the load applied to the film at the time of revolution;
(b) it thereby lengthens the lifetime of the film, i.e., the time that elapses before the film fractures;
(c) it suppresses the above-described problems of the heater; and
(d) it enables high-rate fixing exceeding 24 ppm.
The ceramic heater offered by the present invention is used for a toner-fixing unit, which comprises:
(a) a heating cylinder provided with a ceramic heater along the longitudinal direction of its outer surface;
(b) a continuous heat-resistant film revolving along the periphery of the heating cylinder; and
(c) a pressure roller that forms a nip portion together with the heat-resistant film at the outer region opposite to the ceramic heater on the heating cylinder and that rotates to revolve the heat-resistant film at the same speed as the peripheral speed of the pressure roller.
The toner-fixing unit feeds a copying sheet to the nip portion by the revolution of the heat-resistant film and the pressure roller and fixes a toner image on the copying sheet by the pressure of the pressure roller, and the heat from the ceramic heater on the heating cylinder.
In order to achieve the foregoing object, the ceramic heater of the present invention comprises a ceramic base material, a heating element, and current-feeding electrodes. The heating element and current-feeding electrodes are made of silver or silver alloy and formed on the ceramic base material. In the ceramic base material, at least one part of the face that contacts the heat-resistant film is curved when viewed from a direction perpendicular to the feeding direction of the copying sheet.
The method for manufacturing the ceramic heater of the present invention is a method for manufacturing the ceramic heater to be used in the foregoing toner-fixing unit. A ceramic base material is extrusion-formed in such a manner that at least one part of the face that contacts the heat-resistant film is curved when viewed from a direction perpendicular to the feeding direction of the copying sheet. After the formed body is sintered, a silver or silver-alloy heating element and silver or silver-alloy current-feeding electrodes are formed on the ceramic base material at the face where the ceramic base material contacts the heat-resistant film or at the reverse side of the face.
The present invention reduces the degree of deformation of the heat-resistant film to lighten the load applied to the film at the time of revolution, so that the fracture of the film can be prevented. As a result, the heat-resistant film can be used at a fixing rate exceeding 24 ppm, which cannot be realized by the prior art. Consequently, the present invention enables the realization of a fixing system that uses a ceramic heater and a heat-resistant film, that consumes fixing power as low as the conventional system, and that can fix images at a high rate exceeding 24 ppm.